V.S. Veerasamy

HIGHLY TETRAHEDRAL, DIAMOND-LIKE AMORPHOUS HYDROGENATED CARBON PREPARED FROM A PLASMA BEAM SOURCE

A highly tetrahedral, diamond‐like form of hydrogenated amorphous carbon (a‐C:H) with density of 2.9 g cm−3, sp3 fraction of 0.75, and hardness of 61 GPa, the highest values so far attained from a hydrocarbon source gas, has been deposited from acetylene in a novel plasma beam source. The ion energy for highest sp3 fraction is measured to be about 100 eV per C atom, similar to that for a‐C, indicating that the subplantation deposition model of a‐C also describes a‐C:H.

n-type doping of highly tetrahedral diamond-like amorphous carbon

The ability to control the electrical resistivity of semiconducting tetrahedral amorphous carbon (ta-C) films is reported. Phosphorus is incorporated during film growth from a plasma formed with a cathodic vacuum arc by using a phosphorus-containing carbon cathode. By changing the P content in the cathode from 0-1% mass the room temperature resistivity of the films can be changed from 107 Omega cm to 5 Omega cm. Electron energy-loss spectroscopy shows that incorporation of P does not change the amorphous tetrahedral nature of the carbon films deposited by the vacuum arc method.

Optical and electronic properties of amorphous diamond

Abstract The optical and electronic properties of highly tetrahedral amorphous diamond-like carbon (amorphous diamond, a-D) films were investigated. The structure of the films grown on silicon and glass substrates, under similar deposition conditions using a compact filtered cathodic vacuum arc system, are compared using electron energy loss spectroscopy (EELS). Results from hydrogenation of the films are also reported. The hydrogenated films show two prominent IR absorption peaks centred at 2920 and 2840 cm−1, which are assigned to the stretch mode of the CH bond in the sp3 configuration on the CH3 and CH sites respectively. The high loss EELS spectra show no reduction in the high sp3 content in the hydrogenated films. UV and visible transmission spectra of a-D thin films are also presented. The optical band gap of 2.0–2.2 eV for the a-D films is found to be consistent with the electronic bandgap. The relationship between the intrinsic compressive stress in the films and the refractive index is also presented. The space charge limited current flow is analysed and coupled with the optical absorption data to give an estimate of 1018 cm−3 eV−1 for the valence band edge density of states.

Hydrogen-free amorphous carbon preparation and properties

Abstract Amorphous carbon containing very little hydrogen and having a highly tetrahedral character is deposited by filtered cathodic arc. This amorphous carbon shows photoconductivity under white light illumination. It is slightly p-type in pure form but becomes n-type when doped with nitrogen. Electron diffraction shows that nitrogen levels of less than 1% do not distort the tetrahedral network, but at higher concentrations nitrogen destabilises the tetrahedral structure. Plasmon spectroscopy shows the presence of a 1 nm “defective” surface layer even in the pure material which has implications for device applications. A junction field effect transistor structure is proposed as a possible device.

Substitutional nitrogen doping of tetrahedral amorphous carbon

Abstract Amorphous carbon films prepared by filtered cathodic arc deposition have been studied by electron-energy-loss spectroscopy. It is shown nitrogen concentrations in the film of up to 30at.% can be controllably introduced. For nitrogen concentrations of up to 2–3 at.% the films are primarily sp3 bonded, whilst films with higher nitrogen concentrations are primarily sp2 bonded. It is shown that the nitrogen is substitutionally incorporated into both the sp3 and the sp2 carbon networks. The presence of nitrogen in the films with a high sp3 fraction leads to a sensitivity to conversion to the sp2−rich form in a 300 keV electron beam.

Electronic density of states in highly tetrahedral amorphous carbon

Highly tetrahedral amorphous carbon (ta-C) films and its hydrogenated form (ta-C:H) have been deposited using a filtered cathodic arc. In this paper, the optical and electronic band-gaps of amorphous diamond and its hydrogenated form ta-C:H, are shown to be in the range 2 eV. The H content of ta-C:H is found to be about 3 at.% using elastic recoil detection analysis (ERDA) and electron energy loss spectroscopy (EELS) is used to show that ta-C:H does keep its tetrahedral structure. Both materials are found to be stable at temperatures up to 1000 K. The room temperature conductivity of 10−7−10−8 Ω−1cm−1 is shown to be thermally activated, with activation energies in the range 0.2–0.3 eV. A detailed study made from the current-voltage characteristics of ta-C/p-Si heterojunctions shows that current flow is space-charge limited (SCLC) and influenced by bulk traps. The density of states profile, N(fE), in the region of the quasi-Fermi level is calculated using the differential method and found to be of the order 1018 cm−13 eV−1. Optical absorption data is coupled with the electrical results to confirm that the valence band density of states is of the order 1021 cm−13 eV−1. It is also shown that there is a reduction of nearly one order of magnitude in the N(E) at the quasi-Fermi level upon hydrogenation. SCLC measurements are complemented with surface admittance measurements (SAM) which yield a value of 1011 cm−2 eV−1 for the interfacial density of states at the ta-C/p-Si junction. A decrease in Nss is also observed at the ta-C:H/p-Si interface in agreement with the SCLC results.

Doping of highly tetrahedral amorphous carbon

The doping of amorphous carbon films with over 80% sp3 bonding is examined. It is shown that n-type doping with P and N is possible. p-type doped with B has been unsuccessful. The electronic properties of this form of amorphous carbon which has an optical band-gap of 2eV is taken as being governed largely by the π and π∗ states which arise due to the much smaller fraction of sp2 bonded C. With low levels of N doping there is evidence for compensation of p-type defects in the undoped material. It is thought that dopants which have deep levels in diamond appear as shallow dopants in tetrahedral amorphous carbon due to the presence of the π states.

Photoresponse characteristics of n‐type tetrahedral amorphous carbon/p‐type Si heterojunction diodes

The photovoltaic behavior and spectral response of n‐type (nitrogen‐doped) tetrahedral amorphous carbon (ta‐C)/p‐type crystalline silicon heterojunction photodiodes are reported. Abrupt step junction type characteristics are observed with ta‐C films ranging in thickness from 40 to 160 nm. The photovoltage increases and the prominent peak in responsivity shifts from 800 nm to longer wavelengths of 1000 nm as the doping in the ta‐C films is increased, indicative of a widening of the depletion region in the Si. Use of the responsivity vs wavelength data with the Donnelly and Milnes model for an abrupt heterojunction is successful in predicting the depletion width in the Si and the doping level in the higher doped ta‐C. The latter value coupled with the x‐ray photoemission spectroscopy derived N concentration in the ta‐C films enables an estimation of the doping efficiency. Secondary ion mass spectroscopy studies also confirm the abrupt nature of the junctions.

Photoconductivity in highly tetrahedral diamondlike amorphous carbon

A photoconductive effect is observed in tetrahedral amorphous carbon (ta‐C) or amorphous diamond films with a high proportion of sp3 bonding and a structure analogous to amorphous silicon. The spectral response is shown to peak at around 750–800 nm and extend into the near UV region. The maximum photoresponse coincides with the optical absorption edge. The quantum efficiency of 300 nm thick films with interdigitated electrodes is over 10% in the wavelength range 450–800 nm.

Analysis of the ellipsometric spectra of amorphous carbon thin films for evaluation of the sp3-bonded carbon content

Abstract Using spectroscopic ellipsometry (SE), we have measured the optical properties and optical gaps of a series of amorphous carbon (a-C) films ∼ 100–300 A thick, prepared using a filtered beam of C + ions from a cathodic arc. Such films exhibit a wide range of sp 3 -bonded carbon contents from 20 to 76 at.%, as measured by electron energy loss spectroscopy (EELS). The Tauc optical gaps of the a-C films increase monotonically from 0.65 eV for 20 at.% sp 3 C to 2.25 eV for 76 at.% sp 3 C. Spectra in the ellipsometric angles (1.5–5 eV) have been analyzed using different effective medium theories (EMTs) applying a simplified optical model for the dielectric function of a-C, assuming a composite material with sp 2 C and sp 3 C components. The most widely used EMT, namely that of Bruggeman (with three-dimensionally isotropic screening), yields atomic fractions of sp 3 C that correlate monotonically with those obtained from EELS. The results of the SE analysis, however, range from 10 to 25 at.% higher than those from EELS. In fact, we have found that the volume percent sp 3 C from SE using the Bruggeman EMT shows good numerical agreement with the atomic percent sp 3 C from EELS. The SE-EELS discrepancy has been reduced by using an optical model in which the dielectric function of the a-C is determined as a volume-fraction-weighted average of the dielectric functions of the sp 2 C and sp 3 C components.